This invention relates to a closed-loop servo control, and more particularly to a control having closed-loop terms which are adaptively tuned and selectively activated to provide both stable operation and improved performance.
Servo controls are employed in a wide variety of applications for maintaining the actual value of a controlled parameter in agreement with a desired or commanded value. In many applications, the control may be characterized as closed-loop, in that the actual value of the controlled parameter is measured and compared to the desired value to form an error signal, which in turn, determines the magnitude and direction of effort required to bring the actual value into agreement with the desired value. For example, in an application in which the position of a motor-driven actuator is to be controlled, the actual position of the motor or actuator is typically measured to form a position error signal, which in turn, is used to control the motor direction and torque, with the objective of driving the error signal substantially to zero. Such a control is generally referred to as a proportional control since the control effort is determined in proportion to the measured error. Stability and performance can be enhanced by additionally adjusting the effort in relation to the integral of the error (so-called integral control) and/or the derivative of the error (so-called derivative control). In general, integral control is used to eliminate small errors that persist over a period of time, while derivative control is used to increase the effort in response to sudden increases in the error signal.
While the above-described control techniques can be used to achieve reasonably fast and stable servo performance, calibrating the closed-loop gains can be a very difficult and time consuming process, particularly in applications involving a wide range of operating conditions and non-linear loading. In an automotive environment, for example, the control has to be designed to achieve reasonably fast and stable performance over wide ranges of ambient temperature and operating voltage. One approach is to tune the control gains to ensure stable operation in a worst-case condition, such as high temperature/high voltage. However, this approach is a compromise by definition, and results in poor overall performance. Another approach is to adjust the control gains on-the-fly by table look-up based on various system measurements. However, this approach requires increased system expense and a considerable calibration effort that has to be repeated if the system design changes. Even under the most favorable conditions, the performance of the control is typically compromised to ensure stability.
Accordingly, what is needed is a closed-loop servo control that provides both stable operation and improved performance, with minimal system expense and calibration effort.
The present invention is directed to an improved closed-loop control of a servo motor in which multiple closed-loop terms are adaptively tuned and selectively activated to achieve both stable operation and improved performance, as compared to conventional closed-loop controls. According to the invention, the motor command includes a first term proportional to an error signal, a second term based on the integral of the error signal, and a third term based on the rate of change of the measured feedback signal, and representing the kinetic energy of the controlled parameter. The first term is continuously active, whereas the second term is only activated when the rate of change of the measured feedback signal is below a reference rate of change, and the third term is only activated when the error signal is within a reference window, thereby allowing relatively high gains while ensuring stable operation. In applications involving an oscillatory load, changes in system response are detected based on variation in the rate of change of the measured feedback signal for purposes of adaptively adjusting the predefined gains and references, thereby eliminating the need to measure the various ambient, system or load parameters that affect system performance and stability. Various other features of the control include a technique for reducing stress and energy consumption during anticipated stalling, processing techniques for sampling the feedback variable and identifying background noise, and biasing the system toward zero error during static operation.